Slide guitar is a particular method or technique for playing the guitar. The term slide refers to the motion of the slide along the strings. Instead of altering the pitch of the strings in the normal manner (by pressing the string against frets), an object called a "slide" is placed upon the string to vary its vibrating length, and pitch. This slide can then be moved along the string without lifting, creating smooth transitions in pitch and allowing wide, expressive vibrato.

Slide guitar is most often played (assuming a right-handed player and guitar):

With the guitar in the normal position, using a slide on one of the fingers of the left hand.

With the guitar held horizontally, belly-up, using a metal bar called a "steel" ("slides" generally fit around a finger) held with the hand and wrist above the frets, fingers pointing away from the player's body; this is known as "lap steel guitar". This same technique is used to play pedal steel guitar and the "Dobro" resonator guitar used in Bluegrass music.

Most recently lap style slide has been reborn via artists like Ben Harper, Jack White, Sean Kirkwood and Xavier Rudd - both Harper and Rudd are players of Weissenborn slide guitars, the former using original early 1900s instruments long with modern day variations such as his own co-designed Asher signature model, the latter using modern reproductions.

Wooden resonator guitar played with a steel, angled to form a chord unavailable from straight open tuning.

An ordinary guitar, either electric or acoustic, can be used for playing slide. Often the strings are raised a little higher off the fingerboard than they would be for ordinary guitar playing. This is done especially if the free fingers are not going to be used for fretting. An extension nut may be used to achieve the higher string height at the peghead end of the fingerboard. This is just a normal nut, with the slots filed less deeply, and often in a straight line rather than following the radius of the fretboard. The lap steel and the pedal steel are guitars that have evolved especially for playing slide in the horizontal position. Resophonic or resonator guitars have often been employed for slide playing, typically held horizontally. They are sometimes known as Dobros after the Dopyera brothers, whose company first made them. National is another brand. In resonator guitars, rather than the sound being produced by the body's hollow, a special bridge transfers the vibrations from the strings to a metal cone placed inside the body.

A slide can be made with any type of smooth hard material that allows tones to resonate. The slide's weight (in terms of density and wall thickness) cause differences in sustain, timbre, and loudness, while the surface structure and material affect tonal clarity and timbre. Square, beveled or rounded edges may allow a player to apply different techniques, while tapered rather than straight sides may help improve control and cause less damping. Pedal steel players may prefer using tonebars, which have one capped end. One recent development is the rise of hybrid slides. Glass Moonshine slides are made of glass, but have a porous ceramic interior that helps prevent slipping; other slides have been designed to reduce the weight of brass or porcelain slides by using a lightweight interior, while still others are made of glass on the front and of metal on the back to allow easy switching. One can use a solid metal bar or rod, laid across the strings of the guitar and held by the fingers of the fretting hand being laid on it to either side, parallel to it. Pipes, and stones have also been used to good effect, as have rings and spoons. Even a knife can reportedly be used: "As he played, he pressed a knife on the strings of the guitar in a manner popularised by Hawaiian guitarists who used steel bars. The effect was unforgettable." ―W. C. Handy on his first hearing slide guitar, a blues player in the Tutwiler, Mississippi train station.

Nick Manoloff was a pioneer in the making, patenting and commercially marketing slides as early as 1937. In 1989, Terrie Lambert invented the Moonshine (ceramic) slide that produces a timbre in between that of brass and glass, and the Mudslide (porcelain) slide, which like brass slides is quite heavy, producing richer, fuller and resonating tones with more harmonics. As a result, they are often used in blues music. The Moonshine and Mudslide slides are glazed on the outside but porous on the inside so that finger moisture is absorbed, preventing slippage. Metals such as stainless steel, chrome and aluminium cause a bright penetrating sound and are mostly used with electric guitars, among others for rock music. Necks cut from bottles, segments of copper or PVC plumbing pipe, small glass medicine bottles, and even deep length wrench sockets are commonly used by those who do not choose to buy a slide. Phil Hoolahan invented the unique Hoolahan slide that totally sets your fingers free to play traditional style guitar using all four fingers while wearing a slide that can be used at anytime to play haunting slide riffs.

The slide is pressed against the strings—lightly, so as not to touch the strings to the fretboard, and parallel to the frets. The pitch of the strings can then be continuously varied by moving the slide up and down the fingerboard. The usual limitation in fretted guitar playing of twelve pitches per octave does not apply. The technique lends itself to glissandi (swoops up or down to a note); in addition it has the ability to evoke sounds of the human voice or natural noises. Another strength of the technique is its vibrato, which is easily achieved by oscillating the hand so that the slide goes quickly back and forth.

The major limitation of slide playing is of course that only one chord shape is available: whatever the strings happen to be tuned to going straight across. Many slide guitarists will still use their free fingers to fret the strings if they want to employ that sound as well. Using the free fingers opens up the possibility of playing chord shapes other than the straight line given by the slide. One strategy is to use the free fingers for rhythm work, and intersperse this with lead phrases played with the slide.

The guitar may be held in the normal guitar-playing position (that is, with the face of the guitar more-or-less vertical) or it may be held flat, with the face of the guitar horizontal. In the latter case the guitar may sit flat in one's lap or on a stool, face up, or held in this position by a strap, and played standing up. If holding the guitar in the normal vertical position, it is more common to use the tube type of slide. In the horizontal approach, solid bars are more commonly used, and the grip is overhand: the hand is not wrapped around the slide, the index finger is nearest the bridge, the little finger nearest the nut, fingers pointing away from the chest.

Usually, a slide player will use open tuning, although standard tuning is sometimes used. In open tuning the strings are tuned to sound a chord when not fretted. The chord tuned to is most often major. Open tunings commonly used with slide include Open D or "Vestapol" tuning: D-A-d-f#-a-d; and Open G or "Spanish" tuning: D-G-d-g-b-d. Open E and Open A, formed by raising each of those tunings a whole tone, are also common. These tunings can be traced back to the 19th century through the banjo, predating the Hawaiian guitar. Other tunings are used as well.

Slide guitar is most often fingerpicked, with or without plastic or metal picks on the thumb and fingers. However some players use a flatpick (plectrum).

The bottleneck or tube type of slide is usually worn over the ring (3rd) or little (4th) finger, while others wear it over the middle (2nd) finger. Wearing it on the 4th finger has the advantage of leaving one more finger free to fret notes if desired. However some players feel that they get better control using the ring or middle fingers. Most instructors recommend letting one or more of the fingers behind the slide rest lightly on the strings to help mute unwanted vibrations.

A relatively new technique,[citation needed] expanding the musical range and sonic capabilities of slide guitar, is the system of double slide guitar. It was invented by Brian Cober,[5][non-primary source needed] a Canadian blues musician. In double slide, the first slide is placed on the middle finger (usually a modified steel bar that can be put on the finger), and a modified thumb slide is put on the thumb that is able to cover two strings. Double slide is meant to be played on a six-string lap guitar (or a regular six-string guitar modified with the strings raised for high action like a lap guitar), usually tuned to open E tuning. The double slide guitar system enables the player to play chords not heard in open tunings, such as minor chords, dominant seventh chords, etc. and provides a greater use of technique in soloing.[6]Will Ray of the Hellecasters uses a similar technique, wearing "stealth" pinky-type slides on either hand.

The following samples give an impression of the various styles of slide guitar. First is Robert Johnson's "Traveling Riverside Blues", one of the best-known examples of Delta blues slide guitar. Second, a part of Duane Allman's solo from Derek and the Dominos's "Layla" is included, to give an impression of highly acclaimed slide work in rock music.

The Magic and Mystery of Slide Guitar - an exhibit curated by the Museum of Making Music, National Association of Music Merchants, Carlsbad, CA – detailing the history and evolution of slide guitar technique.